1. Field of the Invention
This invention relates to an image forming apparatus utilizing the electrophotographic recording method, such as a laser printer, a copier or a facsimile apparatus, and particularly to an image forming apparatus provided with a plurality of image bearing members and suitable for use in the in-line method wherein images formed on the image bearing members are sequentially superimposed on the same transferring member to thereby form images of a plurality of colors.
2. Related Background Art
Various color image forming apparatuses utilizing the electrophotographic recording method to form images of a plurality of colors have heretofore been devised and some of them have been put into practical use.
The image forming apparatuses of the electrophotographic recording type are excellent in that the recording speed can be made high, over image forming apparatuses of other recording types such as the ink jet type in which ink droplets are directly blown against recording paper to thereby form an image, and the silver salt photographic type in which image exposure is effected on a photosensitive chromophoric material to thereby effect recording, and are achieving distinction from those other recording types for the market needs desiring a higher speed.
As a typical example of color image forming apparatuses utilizing the electrophotographic recording method, there is, for example, a color image forming apparatus of a type containing a rotatable developing apparatus therein. The color image forming apparatus of this type is provided with a rotatable body (rotatable developing apparatus) therein, and has an apparatus construction in which developing devices of four colors, e.g. yellow, magenta, cyan and black, are disposed along the rotational peripheral surface of the rotatable body, whereby electrostatic latent image bearing bodies (photosensitive bodies) are sequentially developed with developers (toners) of the respective colors.
In the image forming apparatus adopting such a rotatable developing apparatus, the step of visualizing electrostatic latent images for the respective colors formed on a common photosensitive body as toner images at a predetermined developing position by the respective developing devices, and transferring such toner images onto sheet-like recording paper (transferring material) such as paper each time the toner image is obtained is repeated. By the repetition of such transferring step, toner images of a plurality of colors are formed.
Also, as another example, there has been devised an apparatus of a type in which toner images of respective colors are sequentially selectively superimposed on the surface of a photosensitive body to thereby form toner images of a plurality of colors on the surface of the photosensitive body, whereafter the toner images are collectively transferred to recording paper.
There is also an image forming apparatus of the so-called in-line type in which by the use of a plurality of photosensitive bodies, toner images of respective colors are discretely formed by developing devices of the respective colors, and the toner images are sequentially transferred from the respective photosensitive bodies onto a transferring member and the transferring member is conveyed to thereby form toner images of a plurality of colors.
There is also a type in which toner images of respective colors are sequentially superimposed not directly on recording paper but on an intermediate transferring member (transferring member) to thereby form toner images of a plurality of colors, whereafter the toner images are collectively transferred to the recording paper.
The above-described typical types of the color image forming apparatus utilizing the electrophotographic recording method have both merits and demerits, but from the viewpoint of the higher speed progressing conjointly with the recent market needs, the in-line type is at advantage and many products have been put into practical use by this type.
FIG. 9 of the accompanying drawings is a side view schematically showing an example of the image forming apparatus adopting the in-line type, more particularly the main internal construction of an image forming apparatus of a four-drum multiple transfer type (hereinafter referred to as the in-line color printer).
The in-line color printer 400 has photosensitive drums which are electrostatic latent image bearing members disposed in opposed relationship with developing devices containing toners of respective colors therein and successively disposed in the direction of conveyance of a transferring member, and sequentially transfers toner images of the respective colors developed on the respective drums by the developing devices to recording paper conveyed by a transferring belt or the like, and obtains a full color image by toners of four colors, i.e., yellow, magenta and cyan which are three primary colors, plus black.
In FIG. 9, an endless transferring belt 406 is passed over a driving roller 407 and a driven roller 410, and is rotated in the direction of arrow, and four photosensitive drums 401a to 401d are disposed in series in opposed relationship with the transferring belt 406. Generally, to obtain a high quality of image in the apparatus of this type, it is important to reduce the color misregistration among the images of a plurality of colors formed by superimposition, and it is preferable to make the station spacings among a plurality of image forming means (image forming stations), in other words, the spacings among the photosensitive bodies, equal to each other, and the disposition accuracy thereof is important.
It is also effective for the reduction in color misregistration to make the circumferential length of the driving roller and the spacings among the photosensitive bodies equal to each other.
Also, the image forming stations 409a, 409b, 409c and 409d for forming images of respective colors are comprised of photosensitive drums 401a to 401d, charging apparatuses 402a to 402d, exposing apparatuses 403a to 403d, developing devices 404a to 404d and cleaning apparatuses 405a to 405d, and these are disposed around the respective photosensitive drums.
In the image forming stations 409a, 409b, 409c and 409d, developer images of the respective colors are superimposed on a recording material conveyed on the transferring belt 406 by yellow (Y), magenta (M), cyan (C) and black (Bk) developers to thereby form a multicolored image. Also, the image forming stations of the respective colors have constructions substantially similar to one another with the exception that the developers of different colors are contained in the developer containing portions of the developing devices 404a to 404d. 
The image forming operation will hereinafter be described. A laser beam (information light including predetermined image information) modulated in conformity with image data from a host apparatus such as a personal computer is applied from the exposing apparatuses 403a to 403d to the surfaces of the photosensitive drums 401a to 401d uniformly charged by charging rollers which are charging apparatuses 402a to 402d, whereby desired electrostatic latent images are obtained for the respective colors. These latent images are reversal-developed on the predetermined developing regions of the rotational peripheral surfaces of the photosensitive drums by the developing devices 404a to 404d disposed in opposed relationship with the aforementioned predetermined developing regions and containing toners of the respective colors therein, and are visualized as toner images. These toner images are electrostatically multiplexly transferred in order of Y, M, C and Bk in transferring nip portions by the transferring apparatuses 408a to 408d to recording paper P fed by a paper feeding mechanism, not shown, i.e., the recording paper P conveyed by a conveying mechanism and entering from the right as viewed in FIG. 9, and a resultant color toner image is melted and fixed by a fixing apparatus, not shown, and is permanently fixed on the recording, paper whereby a desired color print image is obtained.
When a monochromatic image of a single color Bk is to be formed, the image forming means of the other colors than Bk are not operated, and a similar image forming operation is performed with the transferring belt 406 and the photosensitive drums 401a to 401c being spaced apart from each other by a mechanism, not shown.
Also, after the transfer, any toners not transferred but residual on the photosensitive drums 401a to 401d are removed by the cleaning apparatuses 405a to 405d such as cleaning blades, and the photosensitive drums are prepared for the next image forming process.
Also, with regard to the developing method, generally any of the method of contact and the method of non-contact with the photosensitive drums may be applied to the in-line color printer, and the toners which are the developers may be of any of the monocomponent type and the two-component type. As an example, mention may be made of the contact developing method by a non-magnetic monocomponent toner.
In the in-line color printer 400 of FIG. 9, the image forming stations 409a to 409d form process cartridges in which the photosensitive drums 401a to 401d, the charging apparatuses 402a to 402d, the developing devices 404a to 404d and the cleaning apparatuses 405a to 405d are made integral with one another and which are detachably attachable to the main body of the in-line color printer 400.
For example, FIG. 10 of the accompanying drawings enlargedly shows a process cartridge (image forming station) 409a for effecting the development by the yellow (Y) toner among the process cartridges shown in FIG. 9. Incidentally, the other process cartridges 409b to 409d for M, C and Bk have the same form and function as those of the process cartridge 409a with the exception that the colors of the developers (toners) contained therein differ from one another.
The in-line color printer 400 of the present example has means for detecting the amount of residual toner in each process cartridge, and when the toner in the cartridge of a certain color has become exhausted and that cartridge has reached its life, it is possible for the user to continuedly use the printer simply by interchanging that process cartridge, and such maintenance by a servicemen that is effected in a copier or the like is unnecessary.
There is also the advantage that the image forming stations form the process cartridges, whereby good images free of any inconvenience can always be stably obtained until the life of the process cartridges expires, the user can easily effect the interchange of the process cartridges.
Now, when a market research regarding the user""s actual use of color image forming apparatuses of the electrophotographic type has been done, it has been confirmed that printed images include various images such as pictorial full color images like photographs, one-point business color images having colors partially attached to portions to be emphasized, and monochromatic images, but the consumption of the black toner is great when viewed in terms of the total quantity of used toners.
Further, there is a demand for minimizing the cost per sheet during the printing of monochromatic images.
However, when as in the apparatus shown in FIG. 9, the quantities of toners in the developing devices are all the same, the frequency of the interchange of the Bk process cartridge is high, and this is cumbersome and leads to a problem in maintenance.
Also, when the image forming means are not in the form of process cartridges, the interchange of the photosensitive drums and the interchange of the charging apparatuses, the developing devices or the toner containers are effected discretely from each other, and the frequency of maintenance becomes still higher.
On the other hand, to form images of high quality stably in various environments, it is necessary to sufficiently agitate and circulate the toners in the developing devices. If there is a problem in the agitation of the developers or the conveyance of the developers to the developing portions, there will arise such a phenomenon that the developers stagnate or adhere and lump (block) and thus, irregular density or streaks will occur to outputted images.
Particularly, when the quantities of developers in the developing devices are to be increased, it is necessary to care about this point.
In view of these points, it would occur to mind to horizontally enlarge the width of the container of the developing device 404d of the image forming station 409d (Bk) shown, for example, in FIG. 9, so as to make the quantity of the toner contained in the developing device 404d great relative to the quantities of the toners of the other colors, but this would widen the distance between adjacent ones of the image forming stations, and would make the in-line color printer 400 itself bulky.
Also, in terms of the quality of image, when the correction of the above-mentioned color misregistration and the ease of the control of the image forming apparatus are taken into consideration, equidistance is ideal as the distance between the stations and to prevent the entire apparatus from becoming bulky, it is preferable that the distance between the image forming stations be as short as possible.
Conversely, it would also occur to mind to extend the container along the direction of gravity (a direction perpendicular to the direction of conveyance of the recording paper), but in this case, it will become difficult to agitate the developer sufficiently, and the so-called packing state that is, the phenomenon that the underlying toner is crushed by the weight of the overlying toner and becomes clogged and hardened, will become liable to occur. Thus, when the adverse effect on images and the disposition of the exposing apparatus 403d are taken into consideration, the extension of the container along the direction of gravity is neither very preferable.
It is an object of the present invention to provide an in-line type image forming apparatus which is low in running cost and easy to maintain.
It is another object of the present invention to provide an in-line type image forming apparatus which prevents a reduction in quality of image and is restrained from becoming bulky even when the capacity of a developer containing portion containing therein a developer of a particular color used in a great deal is made large relative to that of other colors.
It is still another object of the present invention to provide an image forming apparatus which is provided with a plurality of image bearing members and is easy to control and in which it is difficult for the color misregistration of an image to occur.